Tremendous research efforts have been devoted to the development of polymer-based organic photovoltaic (OPV) cells during the last two decades due to projected advantages of these solar cells over their inorganic counterparts, including flexibility, facile processing and manipulation, low weight and low cost. The mechanism by which light is converted into electricity in these OPV devices consists of the following fundamental steps: light absorption, exciton generation, exciton migration, exciton dissociation and charge transport. The bulk heterojunction (BHJ) of regioregular poly(3-hexylthiophene) (RR-P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) represents one of the most successful systems with reproducible efficiencies approaching 5% after careful optimization. Ma et al., Adv. Funct. Mater. 2005, 15, 1617-1622; Li et al., Nature Mater. 2005, 4, 864-868.
To further improve the performance of polymer-based BHJs, one has to carefully address the following issues. First, the HOMO and LUMO energy levels of the donor and acceptor components need to have optimal offset to maximize the attainable open circuit voltage (Voc). Secondly, the active layer should have compatible absorption with respect to the solar spectrum to maximize the efficiencies of exciton generation, which sets the upper limit for the short circuit current Jsc. Finally, the morphology of the active layer, which governs the physical interaction between the donor and the acceptor, should be optimized to promote charge separation and favorable transport of photogenerated charges and to maximize the attainable Jsc and fill factor (FF). Thompson et al., Angew. Chem. Int. Ed. 2008, 47, 58-77; Scharber et al., Adv. Mater. 2006, 18, 789-794.
Fulfilling these requirements presents serious challenges in the design of new semiconductive conjugated polymers to be employed as active donors in polymer-based BHJ photovoltaic devices. For example, a number of low band gap polymers have been developed in recent years in the attempt to increase the device efficiency by improving light harvesting. Brabec et al., Adv. Funct. Mater. 2002, 12, 709-712; Muhlbacher et al., Adv. Mater. 2006, 18, 2884-2889; Peet et al., Nature Mater. 2007, 6, 497-500; Zhang et al., Adv. Funct. Mater. 2006, 16, 667-674; Andersson et al., Appl. Phys. Lett. 2007, 91, 071108/1-071108/3; Slooff et al., Appl. Phys. Lett. 2007, 90, 143506/1-143506/3; Wienk et al., Appl. Phys. Lett. 2006, 88, 153511/1-153511/3; Yao et al., Appl. Phys. Lett. 2006, 89, 153507/1-153507/3; Ashraf et al., Macromol. Rapid Commun. 2006, 27, 1454-1459; Blouin et al., Adv. Mater. 2007, 19, 2295-2300; Blouin et al., J. Am. Chem. Soc. 2008, 130, 732-742.
However, none of them can outperform P3HT in terms of energy conversion efficiency, mainly due to high lying HOMO energy level with regard to the LUMO of the acceptor (usually PCBM), which reduces the Voc, or ill-defined morphology of the active blend, which reduces the Jsc and FF (or both). In our search for new donor materials, polycyclic aromatic moieties drew our attention. Their rigidly enforced planarity would benefit more effective π electron delocalization when incorporated into the conjugated polymer backbone, which would lead to decreased optical band gaps while providing π-π interactions between polymer chains in thin solid films, thereby improving charge carrier mobility in devices. Roncali, J. Chem. Rev. 1997, 97, 173-205; Tovar et al., J. Am. Chem. Soc. 2002, 124, 7762-7769; Tovar et al., Adv. Mater. 2001, 13, 1775-1780; Polycyclic hydrocarbons I and II; Clar, E. Ed. Academic Press: London 1964; Watson et al., Chem. Rev. 2001, 101, 1267-1300; Shklyarevskiy et al., J. Am. Chem. Soc. 2005, 127, 16233-16237.